The present invention relates to an image heating apparatus employed in an image forming apparatus such as a copying machine or a printer. In particular, it relates to a method for assembling a coil to be employed in an image heating apparatus.
In image forming apparatuses such as copying machines or printers, a toner image (image formed of developer (toner) comprising resin, magnetic material, coloring material, and the like) is formed through one of image formation processes, for example, an electrophotographic image formation process, or an electrostatic recording process. In some image forming apparatuses, a toner image is directly formed on recording medium, and in others, it is indirectly formed on recording medium, that is, it is formed on a primary image bearer and then, is transferred onto recording medium. The thus formed toner image on recording medium is thermally fixed to recording medium with the use of a fixing apparatus. Although there are various fixing apparatuses, a heat roller type fixing apparatus has been widely used.
A heat roller type fixing apparatus comprises a fixing roller (heat roller) and a pressure roller. The two rollers are pressed against each other, forming a pressure nip (fixing nip). As they are rotated, recording medium on which an unfixed toner image is borne is passed through the fixing nip, being pinched by the two roller. As recording medium is passed through the fixing nip, heat and pressure are applied to the recording medium and the unfixed toner image thereon. As a result, the unfixed toner image is thermally fused to the recording medium. As for a means for heating the fixing roller, i.e., heat roller, a halogen lamp has been commonly used as a heat source. More specifically, a halogen lamp is disposed within a fixing roller to heat the fixing roller from within the fixing roller, so that the surface temperature of the fixing roller is maintained at a proper level for fixing.
There has been proposed a fixing apparatus which employs a heating system based on electromagnetic induction, as the means for heating a fixing roller, i.e., a heat roller. According to this system, eddy current is generated in an electrically conductive layer provided on the inward side of the fixing roller, by a magnetic flux from an exciter coil, so that the electrically conductive layer is heated by Joule heat to increase the temperature of the fixing roller.
A fixing apparatus employing the above described electromagnetic induction based heating system is characterized in that its heat generating source can be placed very close to the toner image to be fixed, and therefore, it takes a much shorter time for this type of fixing apparatus to increase the surface temperature of the fixing roller to a proper level for fixation, from the temperature level at the startup of the apparatus, than a fixing apparatus employing a halogen lamp. It is also characterized in that its heat transmission path to a toner image from the heat generation source is shorter and simpler, and therefore, it is high in thermal efficiency.
However, in order to efficiently generate heat in the electrically conductive layer of a fixing roller such as the above described one, which employed an electromganetic induction based heating system, that is, the system in which eddy currently was generated in the electrically conductive layer provided on the inward side of the fixing roller by a magnetic flux from an exciter coil to generate Joule heat to heat the electrically conductive layer, so that the temperature of the fixing roller was increased by the Joule heat, the exciter coil was desired to be disposed along the inward surface of the fixing roller. However, such disposition of the exciter coil made the exciter coil complicated in shape, which reduced the manufacturing efficiency for the exciter coil, increasing therefore the production cost for the exciter coil, which was a problem.
The primary object of the present invention is to provide a simple method for assembling an exciter coil into the inward side of the rotational member of an image heating apparatus, so that the manufacturing efficiency for an image heating apparatus can be improved.
According to one of the aspects of the present invention, a method for assembling an exciter coil into the inward side (space) of the image heating rotational member of an image heating apparatus, comprises a step for winding a piece of wire into a flat and spiral coil, and a step for attaching the flatly and spirally wound coil to a supporting member while reshaping the coil so that the curvature of the coil conforms to the curvature of the supporting member.
According to another aspect of the present invention, it is assured that an exciter coil is reliably secured to a supporting member.
According to another aspect of the present invention, an image heating apparatus comprises a rotational member, a coil for generating a magnetic flux, a supporting member for supporting said coil, and a thermally contractible tube for sheathing said coil and supporting member, wherein an image on recording medium is heated by the heat from said rotational member heated by the eddy current generated by the magnetic flux generated in said rotational member by said coil.
Another object of the present invention is to provide an image heating apparatus in which the temperature of said rotational member does not become lower across its longitudinal end portions that its central portion.
According to another aspect of the present invention, an image heating apparatus comprises a rotational member, a coil for generating a magnetic flux, a first supporting portion for supporting said coil, and a second supporting portion for supporting said coil, wherein said first supporting portion is provided with a projection which extends in the direction perpendicular to the direction in which said rotational member moves, and around which said coil is wound, and said second supporting portion supports the longitudinal end portions of said coil, that is, the portions which extend beyond the corresponding longitudinal ends said projection, and is provided with a semicylindrical surface, along which said coil is positioned, and wherein an image on recording medium is heated by the heat from said rotational member heated by the eddy current generated by the magnetic flux generated in said rotational member by said coil.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.